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A fatal fungal disease of ash trees

First confirmed in the UK in 2012, ash dieback (also known as Chalara or Chalara ash dieback) is a disease of ash trees caused by a fungus called Hymenoscyphus fraxineus (formerly known as Chalara fraxinea).

This disease has spread quickly and is now affecting ash trees and woodlands right across the UK, leading to the death of thousands of trees. Ash dieback has already caused widespread damage in continental Europe.

Sadly there is no cure for ash dieback but some ash trees can tolerate or resist infection. Investigating this natural resistance could be the best way to secure the future of the UK’s ash trees.

Ash dieback is a disease that affects ash (Fraxinus) trees, caused by a fungus called Hymenoscyphus fraxineus. The fungus has two stages to its lifecycle – a sexual stage, which helps the fungus spread, and an asexual stage, which is what grows on the tree and causes damage. The fungus blocks water transport in the tree, leading to lesions in the bark, leaf loss and the dieback of the crown. The ash dieback fungus is believed to have originated in Asia. It was first discovered in Europe in Poland in 1992, and is now found widely across the continent. The first confirmed case in the UK came in 2012 and since then it has spread across England and to Scotland, Wales, Northern Ireland, and the Isle of Man.

The main symptoms of Ash Dieback are:

  • Dead branches
  • Blackening of leaves, which often hang on the tree
  • Discoloured stems, often with a diamond-shaped lesion where a leaf was attached
  • Trees may eventually drop limbs, collapse or fall

The symptoms are often easier to spot in mid-late summer, when a healthy ash should be in full leaf. It becomes much harder in autumn, when leaves are naturally changing colour and falling.

Once a tree is infected the disease is usually fatal – but some trees are tolerant or resistant to infection.   Mature ash trees infected by ash dieback may survive for several years but often succumb to a secondary attack by other pests or pathogens, including honey fungus, which can cause butt or root rot and lead to the tree falling.

The disease may spread locally (over tens of miles) by wind dispersal. The reproductive stage of the fungus grows on the previous year’s fallen leaves, producing fruiting bodies that release spores between June and September. These spores are dispersed by the wind and settle on the leaves of healthy trees. If a healthy tree receives a high enough dose of spores, it too will become infected.  Over longer distances, the disease may be spread by the movement of infected ash plants.

Ash dieback is likely to cause significant damage to the UK’s ash population.  Experience from Europe has shown that young trees are often killed quite quickly and while older, mature trees may survive for longer, they are often brought down by secondary infection (e.g. honey fungus).  

There is no cure for ash dieback but some trees are less susceptible to the disease, so investigating this natural resistance could be the best way to secure the future of the UK’s ash trees.   Following advice from woodland ecologists and plant health specialists it has been agreed that ash dieback cannot be controlled in any permanent way. Scientific evidence suggests that the best way to fight this disease is to allow it to spread through the ash population and wait for trees with natural resistance to regenerate woodlands. 

Whilst the proactive ‘slash and burn’ response worked to some degree with Dutch Elm Disease in Brighton and Hove, it would not be appropriate in this case for two reasons: Chalara spores are spread by the wind, so it would be extremely difficult to create an effective barrier; and ash is much more genetically diverse then English elm, so is more likely to develop a natural resistance to the disease.

Due to the risk of falling branches and collapsing trees, infected ash trees growing beside roads and footpaths are likely to pose a significant threat to public safety and will need to be removed for the safety of site users. In these cases, ecological surveys are important to check for the presence of protected species such as badgers and dormice, enabling the appropriate mitigation to be undertaken.  In general, ash trees in the middle of woodland and away from public footpaths will be left for nature to take its course.

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